1. Field of the Invention
The present invention relates generally to methods and devices for analyzing and mapping soil properties within a field. In particular, the present invention relates to methods and devices for collecting and standardizing soil reflectance data.
2. Description of the Related Art
Accurately and affordably mapping soil properties within a field has proven challenging for soil scientists and precision agriculture practitioners. The sampling density needed to capture small spatial scale variability is impractical using conventional sampling and analysis methods. One of the challenges soon to be facing agricultural soil measurements involves accurately identifying soil carbon sequestration levels. This arises out of the need to reduce atmospheric carbon by increasing the amount of carbon stored in the soil. This would involve contracting with growers to sequester carbon in their soils, and would require accurate measurements to verify the amount of carbon stored. What makes measuring changes in soil carbon levels challenging is the expected carbon increase is small relative to the amount of carbon variability within the field. A device that can map soil carbon variability, used in conjunction with a small number of lab-analyzed soil samples, will lead to improved accuracy of carbon maps.
Soil measurements using diffuse near-infrared spectroscopy (NIR) have been shown to relate closely to soil carbon levels. Reflectance in the NIR portion of the electromagnetic spectrum is highly influenced by molecules containing strong bonds between relatively light atoms. These bonds tend to absorb energy at overtones and combinations of the mid infrared fundamental vibration frequencies. The predominant absorbers in the NIR region are the C—H, N—H, and O—H functional groups, making the NIR region ideal for quantifying forms of carbon, nitrogen and water, respectively. In addition, NIR measurements can frequently be related to other properties of interest, including soil pH, calcium and magnesium.
In order to collect reliable field measurements of soil NIR, the device needs to include internal and external calibration mechanisms that insure calibrated measurements. Soil must be presented to the spectrometer with minimal interference from ambient light, dust, mud, or plant residue. Finally, due to the inherently complex nature of spectroscopy, the method of processing on-the-go soil NIR measurements must be incorporated into a system that performs several critical functions.
Shibusawa U.S. Pat. Nos. 6,608,672 and 6,853,937 both claim the excavation of a survey chamber—a cavity in the soil under the window. This may work acceptably at slow speeds in tilled, well-mixed soil of medium texture and moisture, but it's doubtful that it could be effective in a commercial agricultural application, where conditions and speeds are more challenging.
Shibusawa U.S. Pat. No. 6,853,937 also discloses an EC sensor attached to the excavating shoe. However, Shibusawa's EC sensor only provides a single depth of investigation, at the depth of NIR data collection.